This invention relates to high temperature corrosion resistant alloys and articles made of such alloys.
Articles required to work in a high temperature corrosive environment are known to comprise a base structure or substrate made of a nickel-base base alloy, generally providing the necessary mechanical strength for the articles, and a coating providing protection against corrosion. The useful life of the article is normally the life of its coating as measured by the number of hours which the article can spend at a given temperature in a given environment beofore corrosive effects bring about a given degradation of the coating.
The term "high temperature" is applied to temperatures of the order of 1100.degree.-1150.degree. C. The term "corrosive" includes corrosion by sulphidation and oxidation. Such working conditions are encountered for example by turbine blades in gas turbine engines.
The present invention is concerned with nickel-base alloys of the kind containing chromium and aluminium, refractory metals such tungsten, tantalum and molybdenum, and a rare earth such as yttrium. These constituents are present to satisfy various operating requirements of the article. Thus chromium and aluminium provide low temperature strength while the refactory metals provide high temperature strength. All these constituents have an effect, favourable or otherwise, on coating life. Other elements may be present e.g. cobalt, titanium, zirconium, boron and niobium, as well as carbon, but these are rather less relevant to the issues of coating life.
Coating life may be considered broadly under two headings, viz., corrosion resistance and compatibility with the substrate. To ensure compatibility it is desirable for the composition of the coating to be as near as possible the same as that of the substrate except of course for those constituents by which the coating must necessarily differ from the substrate and where problems can arise from diffusion between coating and substrate. One such constituent is aluminium, and it is known to improve corrosion resistance simply by aluminum-enriching a surface layer of the article whose base structure must not be similarly rich in aluminium as this would be detrimental to mechanical strength. The aluminium-rich surface layer or coating may be produced by diffusing aluminium into the surface of the component or by applying to the substrate a coating having the same composition as the substrate except for the aluminium content. The coating may be applied by such known methods as vacuum deposition, plasma spraying or sputtering.
The aluminium content required for resistance against oxidation corrosion may be as high as 30% whereas the base structure may be limited for reasons of mechanical strength to an aluminum content of say 6%. A certain amount of diffusion of aluminium into the substrate is therefore unavoidable but this is not generally regarded as a serious problem.
However, as far as the other constituents are concerned, there are difficulties which place serious limitations on the use of these constituents and which require careful selection thereof if the requirements of substrate compatibility, corrosion resistance, and other factors (to be described) are to be optimised in terms of component life.